1. Field of the Invention
The invention relates to a multi-layer circuit board, more particularly to a multi-layer circuit board with a thickness of 1.2 mm, which can achieve impedance matching to result in reduced high speed signal reflection and reduced electromagnetic interference.
2. Description of the Related Art
FIG. 1 shows a conventional multi-layer circuit board with a thickness of about 1.2 mm. As shown, the conventional 1.2-mm multi-layer circuit board includes: first, second, third, fourth and fifth insulating substrates (C1), (C2), (C3), (C4) and (C5) disposed sequentially one above the other; a first signal wiring layer (Si) disposed on one side of the first insulating substrate (C1) opposite to the second insulating substrate (C2); a ground wiring layer (GND) disposed between the first and second insulating substrates (C1), (C2); a second signal wiring layer (S2) disposed between the second and third insulating substrates (C2), (C3); a third signal wiring layer (S3) disposed between the third and fourth insulating substrates (C3), (C4); a power wiring layer (PWR) disposed between the fourth and fifth insulating substrates (C4), (C5); and a fourth signal wiring layer (S4) disposed on one side of the fifth insulating substrate (C5) opposite to the fourth insulating substrate (C4). The first, second, third, fourth and fifth insulating substrates (C1), (C2), (C3), (C4) and (C5), the first, second, third and fourth signal wiring layers (S1), (S2), (S3), (S4), the ground wiring layer (GND) and the power wiring layer (PWR) are press-bonded to each other to form the conventional circuit board with a thickness of about 1.2 mm. The first and fourth wiring layers (S1), (S4) are adapted to be mounted with components 10 thereon. The third insulating substrate (C3) has a thickness of about 8 mil. Both of the second and fourth insulating substrates (C2), (C4) have a thickness of about 6 mil. Both of the first and fifth insulating substrates (C1), (C5) have a thickness of about 8 mil. Each of the first, third and fifth insulating substrates (C1), (C3), (C5) is made from a polyester prepreg. Each of the second and fourth insulating substrates (C2), (C4) is made from a fibrous core material that contains paper or glass fibers. In this structure, the first wiring layer (S1) has a first resistance (Rs1) with respect to the ground wiring layer (GND). The second wiring layer (S2) has a second resistance (Rs2) with respect to the ground wiring layer (GND) and the power wiring layer (PWR). The third wiring layer (S3) has a third resistance (Rs3) with respect to the ground wiring layer (GND) and the power wiring layer (PWR). The fourth wiring layer (S4) has a fourth resistance (Rs4) with respect to the power wiring layer (PWR). The first and fourth resistances (Rs1), (Rs4) are about 71 ohms. The second and third resistances (Rs2), (Rs3) are about 46 ohms. During layout, electrical connections pass through the wiring layers and the insulating substrates. Such a conventional circuit board structure has certain drawbacks, which are set forth as follows:
(1) Serious reflection during high speed signal transmission: According to the standard theoretical values determined by Intel, the resistance between two adjacent wiring layers for a circuit board during high speed signal transmission is preferably within the range of 55xc2x110%xcexa9, i.e., between 49.5xcexa9 and 60.5xcexa9. However, the resistances of the conventional circuit board fall outside the preferable range recommended by Intel. In addition, there is a difference of 25 ohms between the value of the first and fourth resistances (Rs1), (Rs4) and that of the second and third resistances (Rs2), (Rs3). Such a large difference will result in an impedance mismatch. Thus, when a high speed signal is being transmitted through the conventional circuit board and passes from the first or fourth wiring layer (S1) or (S4) to the second or third wiring layer (S2) or (S3), reflection of the signal will result. The reflection index of the high-speed signal can be calculated as   ρ  =                    Zl        -        Zo                    Zl        +        Zo              =                            Rs1          -          Rs2                          Rs1          +          Rs2                    =      0.21      
When the reflection is serious, the waveform will be distorted considerably, thereby resulting in poor signal quality. In view of the aforesaid, the conventional 1.2-mm multi-layer circuit board is not suited for high speed signal transmission.
(2) Weakened magnetic flux counteraction: As reflection of high speed signals will generate standing waves, which will increase electromagnetic radiation of the high speed signal, the magnetic flux counteraction of the circuit board is weakened, thereby resulting in excessively high electromagnetic interference.
Therefore, the main object of the present invention is to provide a multi-layer circuit board with a thickness of 1.2 mm, which can achieve impedance matching to result in reduced high speed signal reflection and reduced electromagnetic interference.
Accordingly, a multi-layer circuit board of this invention includes: first, second, third, fourth and fifth insulating substrates disposed sequentially one above the other; a first signal wiring layer disposed on one side of the first insulating substrate opposite to the second insulating substrate; a ground wiring layer disposed between the first and second insulating substrates; a second signal wiring layer disposed between the second and third insulating substrates; a third signal wiring layer disposed between the third and fourth insulating substrates; a power wiring layer disposed between the fourth and fifth insulating substrates; and a fourth signal wiring layer disposed on one side of the fifth insulating substrate opposite to the fourth insulating substrate. The first, second, third, fourth and fifth insulating substrates, the first, second, third and fourth signal wiring layers, the ground wiring layer and the power wiring layer are press-bonded to each other to form the circuit board with a thickness of about 1.2 mm. Each of the first and fifth insulating substrates has a thickness ranging from 4.175 to 4.725 mil. Each of the second and fourth insulating substrates has a thickness ranging from 5.7 to 6.3 mil. The third insulating substrate has a thickness ranging from 15.2 to 16.8 mil. The first signal wiring layer has a first resistance with respect to the ground wiring layer. The second signal wiring layer has a second resistance with respect to the ground wiring layer and the power wiring layer. The third signal wiring layer has a third resistance with respect to the ground wiring layer and the power wiring layer. The fourth signal wiring layer has a fourth resistance with respect to the power wiring layer. The first, second, third and fourth resistances are within the range of 49.5 to 60.5 ohms.